Unitary balanced-inductor system



Oct. 19, 1948. H. A. WHEELER 2,451,595

UN-ITARY BALANCED INDUCTOR SYSTEI Filed Dec. 31, 1942 2 Sheets-Sheet 1 TRANSMlTTER FIG. I I 3 HAROLD A. WHEELER ATTORNEY Oct. 19, 1948. H. A. WHEELER UNITARY BALANCED INDUCTOR SYSTEM 2 Shoots-Sheet 2 Filed Dec 3 1942 2 rlllnlldlullljv FI G 5 INVENTOR D WHEELER BY A RNEY Patented Oct. 19, 1948 NlED STATES OFFICE 2,451,596 UNITARY BALANCED-INDUCTOR SYSTEM Application December 31, 1942, Serial No. 470,785

6 Claims.

The present invention relates to unitary balanced-inductor systems and, particularly, to such systems of the type which has a plurality of inductors at least two of which are adapted to be connected in one electrical circuit and others in a second electrical circuit, the inductors being so Wound and disposed relative to each other that alternating current flowing in one electrical circuit will produce a field in the vicinity of all of the inductors but normally will not induce any appreciable voltage in the other circuit until the inductive coupling between the inductors is disturbed by some external condition.

In certain applications, for example in some detectors of buried metallic objects or bodies of ore, it is desirable to provide an inductor system having at least two inductors individually connected in two electrical circuits of the detector system but normally providing no coupling between the circuits. When a magnetic or conductive body is brought into the vicinity of such an inductor system, the coupling between the inductors is modified and electrical energy is transferred from one electrical circuit of the detector system to the other. This transfer of energy between the circuits is used to actuate a suitable device to indicate the presence of the conductive body.

Perhaps the simplest prior inductor system of this nature comprises a pair of inductors crossed at right angles and included in individual electrical circuits of the detector system. Inductors relatively positioned in this manner have substantially zero mutual inductance and consequently are normally not effective to transfer energy between the circuits of the detector system. Another prior arrangement utilizes a pair of parallel inductors close together and connected in individual circuits of the detector system, the inductors being overlapped just sufficiently to provide Zero mutual inductance. Both of these dual-inductor arrangements of the prior art involve some constructional difiiculties'because the inductors cannot be-made coplanar. Furthermore, in the overlapping inductor arrangement, it is impossible to compute the position of the two inductors for zero mutual inductance and it is difficult to maintain such relation if the inductor system must be operated under conditions of varying temperature. The crossedinductor system has the important disadvantages that it is unresponsive to a metallic object on the axis of either inductor and the inductors thereof become appreciably and undesirably coupled due to the presence of the earth for even slight variations of the inductors from the horizontal and vertical positions in which they normally are used.

An additional prior art inductor system includes three inductors of the same size sup- .the same size, cannot be made coplanar to minimize the volume necessarily occupied by the inductor system. There is the additional disadvantage that the inductors of this arrangement cannot be made concentric and, consequently,

. the balance of coupling between the center ina second electrical circuit.

ductor and the end inductors is critically sensitive to movement of any one of the inductors, such as might be experienced during initial construction of the inductor system or by unequal displacements of the inductors with changes of temperature. This inductor system, in common to those heretofore considered, has the additional important disadvantage that the system is in capable of providing a critical indication of the distance from the inductor system of any body which is effective to produce coupling between the inductors thereof, for example the distance below the earths surface of a buried metallic object or a body of ore.

Yet another prior inductor system of this general type utilizes three coaxial inductors, two of which are laid on the ground and connected in one electrical circuit with opposing magnetic fields while the third inductor is positioned on a support above the ground and connected in This arrangement of inductors has the disadvantage that the inductors cannot readily be moved or transported over the ground as a unitary inductor system. Additionally. it is quite difficult to position and arrange the several inductors to provide balanced mutual inductance between the elevated inductor and the two inductors which lie on the ground with the result that this desired condition of balance can only be attained by experiment after each new disposition of the inductors.

It is an object of the present invention, therefore, to provide a new and improved unitary balanced-inductor system which avoids one or more of the limitations and disadvantages of the prior art systems of this nature.

It is a further object of the invention to provide a new and improved unitary balanced-inductor system in which at least two of the inductors are adapted to be connected in one electrical circuit and other inductors in at least one additional electrical circuit and in which the relative positions and parameters of the inductors for substantially zero inductive coupling between the circuits are readily determined by computation accuse which the desired mutual coupling between the inductors does not appreciably change with temperature and is-not critical in respect to either the shape of the inductors or their centering.

It is an additional object of the invention to provide a unitary balanced-inductor system particularly suited for use in a system for locating buried masses of magnetic or conductive material or bodies of ore and one which is effective to determine the position and the depth at which the material or body is buried.

It is yet an additional object of the invention to provide a new and improved unitary inductor system adapted to be carried in proximity to a mass of material having a substantially fiat surface and exhibiting an electrical characteristic, for example magnetic permeability, and one which is not only substantially unresponsive to variations of the distance of the system from the mass of material but additionally has greatly reduced response to undesirable slight departures in the orientation of the system from a predetermined normal orientation relative to the surface of the mass of material.

In accordance with the invention, therefore, a unitary balanced-inductor system comprises a first inductor-having a predetermined shape and diameter 111 and number of turns m, and a, second inductor having the same predetermined shape but having a larger diameter d: and a number of turns m. The second inductor is positioned in fixed coaxial relation to the first inductor and the diameters and numbers of turns of the first and the second inductors are proportioned inaccordance with the relation n1/na=\/da/d1 so that alternating currents of equal magnitudes and opposite phases in the inductors produce a magnetic field which has zero normal component of intensity over a predetermined unbroken three-dimensional surface in space completely enclosing on all sides the smaller of the inductors but completely excluding the larger thereof. The system includes a third inductor having any number of turns and positioned in fixed relation to the first and second inductors and lying substantially in the aforesaid surface, whereby the third inductor has substantially zero inductive coupling to the first and second inductors when carrying the aforesaid alternating currents and in the absence of external concentrated bodies of magnetic or conductive materials but has substantial inductive coupling to the first and second inductors in the presence of such bodies.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings, Fig. 1 is a circuit diagram, partly schematic, of a complete electrical indicating system embodying the present invention; Fig. 2 is a partial cross-sectional view representing a preferred construction of the unitary balanced-inductor system of the Fig. 1 arrangement; and Figs. 3, 4, 5 and 6 represent schematically certain modified forms of the inductor system of the present invention which are suitable for use in the Fig. 1 arrangement.

Referring now more particularly to Fig. 1 of the drawings, there is represented a circuit diagram, partly schematic, of a complete electrical indicating system which includes a unitary balanced-inductor system embodying the present invention in a preferred form. In general, this indicating system includes a transmitter in which may comprise any suitable form of oscillation generator and, if desired, one or more stages of amplification for amplifying the generated oscillations. The system also includes a receiver ii, adapted to receive the oscillations generated by the transmitter iii, which may comprise, in the order named, one or more stages of amplification, a detector for deriving a control signal from received oscillations, and one or more stages of amplification for amplifying the control signal. An indicating device i2 is coupled to the output circuit of the receiver H and is responsive to the control signal derived by the latter. The output circuit comprising output-circuit terminals iii of the transmitter ii) is coupled to a unitary balanced-inductor system id, presently to be described in greater detail. Briefly stated, the inductor system .16 includes a pair of inductors i5, 56 which are serially connected with opposing magnetic fields and are coupled in series to the output-circuit terminals E8 of the transmitter. The inductor system it also includes a third inductor it which is coupled to the input circuit, comprising input-circuit terminals is, of the receiver ll.

Considering briefly the operation of the electrical indicating system just described, the inductor is of the inductor system it is so positioned with relation to the inductors i5 and I6 thereof that, while it is coupled to each of the latter inductors considered alone, it normally is uncoupled to both thereof. Consequently, the oscillations generated by the transmitter ii! and applied to the inductors i5 and is normally are not coupled through the inductor E8 to the input circuit of the receiver H. No control signal is thus developed by the receiver ii and no indication is, of course, provided under such conditions by the indicating device i2.

Now assume that a metallic object which exhibits electrical conductivity or magnetic permeability is brought into the vicinity of the balanced-inductor system i i. The metallic object unbalances or changes the coupling between the inductors of the latter with the result that the oscillations generated by the transmitter iii are now applied to the input circuit of the receiver ii and the control signal derived from the received oscillations is applied to the indicating device iii. The resultant indication provided by the indicating device indicates the presence of the metallic object in the vicinity of the inductor system id.

Referring now more particularly to the portion of the indicating system embodying the present invention, the unitary balanced-inductor system 14 includes, as previously stated, the pair of inductors i5, I6. These inductors are positioned in fixed relation and have the same shape but substantially difl'erent sizes. The terms shape and shape factor, as applied to the inductors of the system H, are used throughout the present specification and claims in the usual and widely accepted technical sense with which the terms are used in the art. The effect of the shape or shape factor of an inductor both on its own value ofself-inductance and on the value of its mutual inductance with another inductor of similar shape and shape factor was early analyzed and carefully explained at length in the United States National Bureau of Standards publication Radio Instruments and Measurements, Circular C74, pages 242-311, inclusive (edition of March 10, 1924--reprinted January 1, 1937). As between two coupled inductors, the two inductors are said to have the same shape factor within the meaning of the present specification and claims insofar as the coefficient of mutual coupling between them is concerned if they have the same ratios of all dimensions that have a significant effect on the value of their mutual inductance. For example, if the winding cross section of one inductor has a maximum dimension which is much less than the winding diameter, the shape and size of the cross section of the inductor have no appreciable efiect on its mutual inductance with another inductor and, hence, only its turns shape (i. e. the shape of its average or mean turn) is important in comparing its shape with that of another inductor. Again, by way of example, if the maximum dimension of the winding cross section of one inductor is not small with relation to the inductor diameter, then the shape and size of the cross section have a substantial effect on its mutual inductance with another inductor so its ratio of maximum cross-sectional dimension to inductor diameter is important in comparing its shape with that of another inductor. Two single-layer cylindrical inductors are thus said to have the same shape when they have the same ratio of length to diameter.

Specifically, the inductors l5, it have the same turns shape or ring shape of small cross section and as shown are planar circular inductors of difierent diameters and are positioned in fixed coaxial coplanar relation. As previously mentioned, the inductors l and 36 are connected in series-opposing relation; that is, are connected in series with opposing magnetic fields. In accordance with the invention, the inductors l5 and 65 have numbers of turns proportioned n the inverse ratio of the square root of their diameters so that alternating currents of predetermined relative magnitudes and phases produce a magnetic field which has Zero normal component of intensity over a predetermined surface in space. In, the particular arrangement of balanced-inductor system shown, the inductors 55 rent fiows through both of the inductors to produce a magnetic field which has zero normal component of intensity over a spherical surface in space, which surface has a diameter equal to the square root of the product of the diameters of inductors i5 and lb.

The third inductor it has the same shape as the pair of inductors 55, M5, for example a planar circular shape, and has a size intermediate that of the pair of inductors l5, It. The third inducfor H3 is shown as fixedly positioned in coplanar coaxial relation with respect to the pair of inductors l5, it. It will, however, later be apparent that this coplanar arrangement of inductor i8 is a preferred, rather than an essential,

6 form by virtue of se eral advantages which relate to the physical construction of the inductor systern. Consequently, the inductor I8 is inductively coupled to each of the inductors l5, l8 but lies substantially in the aforementioned predetermined surface of zero normal component of magnetic-field intensity of the latter inductors and, hence, has substantially zero inductive coupling to both thereof. It may be noted at this point that this is likewise true for any inductor of any shape which lies substantially entirely on the surface of zero normal component of magmay include any desired number of inductors similar to the inductor l8 and all such inductors, if positioned in the manner described, have substantially zero inductive coupling to both of the inductors l5 and Hi. When the third inductor i8 is coplanar and coaxial with relation to the inductors l5 and i6 as described, the diameter of the third inductor is equal to the geometric mean of the diameters of the inductors l5 and I6; that is, to the square root of the product of the diameters thereof. With the inductors l5, l6 and i8 positioned as described, all have a common center; that is, are in concentric relation.

The concentric coplanar arrangement of the inductors l5, l6 and l8 of the inductor system 14 is one which does not require critical centering of the inductors. This is a great advantage in that it facilitates the construction of the induct/or system, particularly insofar as it concerns estabother so that the inductor l8 has substantially zero inductive coupling to both of and IS. The circular shape of the inductors i5, i6 and is has the additional advantage that the inductors are not very critical as to shape, whereby temperature variations have little effect on the desired cancellation of mutual inductance if all of the inductors are constructed of the same material and are maintained at suhstantially the same temperature.

This insensitiveness of the inductor system H to exact centering of the inductors and to slight variations of the relative shapes thereof permits the improved construction of Fig. 2, which represents a cross-sectional view of one-half of an inductor system embodying the Fig. 1 arrangement of the present invention. Elements of Fig. 2 corresponding to similar elements of Fig. 1 are designated by the same reference numerals. Each of the inductors l5, l6 and i8 is wound of a number of turns of very thin insulated copper or aluminum ribbon, aluminum being preferable from the standpoint of light weight. The ribbon winding is cemented together to provide a selfsupporting inductor structure. Each inductor is mounted by flexible supports 2i, preferably of soft resilient rubber in the form of spaced rubber beads, in concentric circular grooves 22, 23 which are cut in two fiat plates of balsa wood or the like. These plates are cemented together with crossed grain to enclose the inductors. As thus arranged. the inductors are fixedly supported in coplanar relationship and are coaxial with relation to their common axis O-O, yet are permitted to move slightly to change their shape or centering with changes of temperature.

It has heretofore been stated that when the alternating currents flowing through the inductors l5, is of the inductor system id have predetermined relative magnitudes and phases, the

inductors l 5, I 8 produce a magnetic field of which the locus of zero normal component of magneticfleld intensity is a predetermined surface in space, and that the third inductor l8 lies substantially in this surface, whereby the third inductor has substantially zero inductive coupling to both of the inductors l5, l6 when the latter carry such alternating currents. It can be shown that this condition holds when the parameters of the inductor system I are proportioned in accordance with the relations:

1u=number of turns in inductor i6, m=number of turns in inductor i5, d1=diameter of inductor i8, d2=diameter of inductor ii, and da=diameter of inductor It.

The mutual inductance between the inductors It and i8 is made equal to that between the inductors l and is by proportioning, in accordance with the invention, the numbers of turns and the diameters of these inductors. Under this condition, Equations 1, 2 and 3 simplify to the following equation, useful in computing the parameters of the inductors it, it and B8 to provide the balanced-inductor system of the present invention:

When the inductors l5, l8 and it of the balanced-inductor system H have numbers of turns and diameters proportioned in accordance with the relations given in Equation 4, the inductor 18 has substantially zero inductive coupling to both of the inductors l5 and I8 when the currents flowing therein have equal magnitude and have relative phases such that the magnetic fields produced by these currents are opposing. This latter condition is fulfilled in the arrangement of Fig. 1 by serially connecting the inductors l5 and It in the same electrical circuit, so that the same current flows through both but in opposite senses.

In general, two circular planar coaxial inductors which carry the same alternating current and are connected with opposing magnetic fields produce a magnetic field of which the locus of zero normal component of magnetic-field intensity is a spherical surface in space. Thus, as shown in Fig. 3, when the inductors I5 and I6 are coplanar concentric inductors with their numbers of turns and diameters proportioned in accordance with the relations expressed in Equation 4, the locus of zero normal component of magnetic-field intensity of their resultant magnetic field is a sphere 24, indicated in broken lines, with its center concentric with that of the inductors l5 and It. The magnetic field of the inductor l5 predominates outside of this sphere 24 while that of the inductor l6 predominates within the sphere,

, the two fields neutralizing each other on the surface of the sphere insofar as the normal component of intensity is concerned. For convenience, therefore, it may be considered that no magnetic flux of either of the inductors 15 or I6 penetrates this spherical surface. For this reason, the spherical surface 24 is hereinafter referred to as a neutral sphere. The neutral sphere has a diameter d2 expressed by the relation:

z=1/ i i This is also the diameter of the inductor it of a coplanar inductor system since the latter inductor is located on the surface of the neutral sphere. In fact, any inductor which is located in any position lying wholly in this spherical surface,

such as the inductor I8 of Fig. 3, has substantially zero inductive coupling to both of the inductors it or it. It will thus be apparent that the dia'rneter of the inductor I8 as given in Equation 5 need apply only to the case where the inductor, i8 is positioned in coaxial coplanar relation with the inductors l5 and it. If the inductor It has a diameter smaller than that given by Equation 5, it can, nevertheless, be positioned to lie on the spherical surface 2%, thus having zero inductive coupling to both of the inductors i5 and it, but in this case cannot be positioned in coplanar relation with these inductors.

In general then, and in accordance with the invention, the two circular coaxial inductors i5 and it are connected in one electrical circuit with opposing magnetic fields and have numbers of turns inversely proportional to the square root of 1 their diameters. The locus of zero normal component of magnetic-field intensity of the resultant magnetic field produced by the inductors is a spherical surface or neutral sphere in space in which either inductor is the image of the other. Any inductor located wholly on the surface of this sphere has no inductive coupling with the electrical circuit which includes the inductors l5 and id.

The invention contemplates that the inductors l5 and it need not be coplanar. Thus as in Fig.4., in which elements corresponding to similar elements of Fig. 1 are designated by similar reference numerals primed, the inductors l5 and I6 are coaxial but are fixedly positioned in parallel planes separated a distance b. Since the inductors have different diameters, they lie on the surface of a cone 25 and alternating currents flowing through the inductors may be adjusted in relative magnitudes and phases, as when the inductors are serially included in the same electrical circuit but are connected with opposing magnetic fields to produce a magnetic field of which the locus of zero normal component of magnetic-field intensity is a spherical surface or neutral sphere 26' with its center at the apex of the cone 25. The third inductor I8 is fixedly positioned with relation to the inductors l5 and I8 and lies substantially entirely on the surface of the sphere 24', whereby it has substantially zero inductive coupling to both of the inductors l5 and i6.

In the particular inductor system of Fig. 4, the inductor l8 has the same diameter as the inductor l5 and is included in a plane parallel therewith. In accordance with the invention, however, the inductor {8' may have a diameter either smaller or larger than that of the inductor l5 and the plane of the inductor l8 may form any desired angle with the plane of the latter. Further, as in the arrangement of Figs. 1 and 3,

any number of inductors similar to the inductor I8 may be used and if located substantially entirely on the surface of the sphere 24 have substantially zero inductive coupling to both of the inductors I and IS.

The particular arrangement of inductors shown in Fig. 4 has the advantage that it facilitates the construction of the balanced-inductor system, particularly in that the inductors l5 and I8, being of the same diameter, may be supported on the same cylindrical support and the inductor l6, being coplanar with the inductor 98', may be easily mounted at one end of the supporting structure. This arrangement has the additional advantage that a geometrical construction may be used to determine the diameter of the inductor l6 if the diameters and spacing of the inductors i5 and I8 are known. This geometrical construction is accomplished in the following manner. Two coaxial circles are first drawn to scale with diameters (12 and d3 corresponding to the diameters of the respective inductors l8 and 15, these circles being spaced a scaled distance b. A sphere 2t, Fig. 4, is then described through these two circles. Next, a cone is described through the circle corresponding to the inductor S5 with the apex of the cone at the intersection of the sphere 26 and the axis 0-0 of the inductors. A circle corresponding to the inductor i6 is then drawn to lie on the surface of the cone and in a plane which includes the circle corresponding to the inductor i d. The neutral sphere 24' is then described through the circle corresponding to the inductor 88' with the center of the sphere at the apex of the cone 25. This construction thus readily determines the scaled diameter of the inductor it and establishes the position in space, relative to the several inductors, of the neutral sphere 2d. The numbers of turns of the inductors l5 and it are given by Equation 4 as in the arrangements previously described.

Each of the arrangements heretofore described possesses the important characteristic that a relatively concentrated body exhibiting magnetic permeability or electrical conductivity and located with its center on the neutral sphere 243, Figs. 1 and 3, or 2 3 in Fig. 4, produces no coupling between the inductors !5, l6 and the inductor it of the Fig. 1 arrangement, or between the inductors 85', l 6 and the inductor id of Fig. 4, whereas any other position of the body in the vicinity of the inductor system does produce such coupling. Thus, when the arrangement of Fig. l is used to locate a buried body of this nature, the general position of the object is determined by the condition of maximum coupling between the pair of inductors l5, l6 and the inductor it. When this has been accomplished, the depth of the buried object is determined by movin the inductor systern it vertically relative to the ground surface until the center of the object lies on the surface of the neutral sphere at which time it produces no coupling between the inductors l5, l6 and the inductor 48 as previously explained. The diameter of the neutral sphere being known for any given inductor system, the depth of the buried object is then readily ascertained. An important application of this nature is in surgery where the system. is used to detect the depth of a metallic object embedded in the body.

If only the position of a metallic body is to be determined, it is preferable so to proportion the parameters of the inductor system i l or M that the diameter of the neutral sphere is fairly small.

On the other hand, where the inductor system I8 is to be used to locate the position and depth of a buried object, the arrangement of Fig. 4 is particularly suitable in that the neutral sphere 24' of this arrangement is larger than the individual inductors of the inductor system I4. In using this arrangement, the inductor system I4 is held with the inductor l5 nearest the buried object in order to determine the general position of the object. Then the inductor system M is reversed and held with the inductors l6 and i8 nearest the object and in such manner that the center of the object is approximately on the axis of the inductor system, The inductor system is then moved axially relative to the body until a sharp indication of zero coupling between the inductors l5, l8 and the inductor i8 is discovered. When this occurs, the buried object is then at a distance from the inductor system l6 corresponding to the point :r on the neutral sphere 2%. It has been found in practice that this indication of the distance of the object from the inductor system it or It is quite critical and, hence, the indicated depth of the buried object is highly accurate.

Due to the geometric relationship between the inductors of the inductor system i l in Fig. 4, there are only certain diameters and spacings of the inductors which have commensurate dimensions. Further, there are certain preferred parameters for the structure of this inductor system by which the numbers of turns of the inductors l5 and i6 have a commensurate ratio which can be satisfied exactly by integral numbers of turns. The following tabulation gives a series of proportions for the preferred parameters in which the inductors l5 and it have a diameter greater than their separation:

Relative Diameters Separation Turns b i /a d2= 'a 1 The following tabulation gives another series of preferred parameters of the inductor system M in which the inductors I5 and it have a diameter less than their separation:

Relative Diameters Separation Turns side of an inductor as indicated in Fig. 5. The

locus of zero normal component of intensity of the magnetic field produced by the inductors l5" and i6" is a geometric surface in space, The inductor l8" need not be of square shape as shown, but may have other shapes so long as it lies approximately in the surface of zero normal component of magnetic-field intensity. Moreover, the inductors l5" and It" need not be coplanar but, as in the arrangement of Fig. 4, may liein parallel spaced planes, the plane of the inductor l8" forming any desired angle with the planes of the inductors l5" and [5".

Fig. 6 represents a modified form of the invention essentially similar to the arrangements of Figs. 1 and 3, similar circuit elements being designated by similar reference numerals, except that the inductors of the inductor system M have certain specific angular relationships relative to each other which render the inductor system particularly suited for use in proximity to a mass of material having a substantially fiat surface and exhibiting an electrical characteristic, for example magnetic permeability or electrical conductivity.

Specifically and by way of example, such a system may be used to locate a metallic object A exhibiting electrical conductivity which is buried in the ground G, the latter exhibiting magnetic permeability and having a relativel fiat ground surface S. A flat ground surface of uniform permeability produces an effect on the inductors of the inductor system M' just as if mirror images 45m, Him and ie'm of the inductors were formed below the ground surface. The strength of these images depends on the permeability of the ground G, but their position below the ground surface S depends only on the surface and homogeneity of the ground. In order that the mirror images lSm, Him of the inductors I5 and i6, respectively, shall not be coupled to the inductor It or, conversely, in order that the mirror image l8'm of the inductor I8 shall not be coupled to the inductors l5 and IS, the inductors l5 and it are fixedly positioned at a predetermined angle relative to the inductor IS. The particular angular relationships required are disclosed and taught in United States Letters Patent No. 1,577,421, granted to Louis A. Hazeltine on March 16, 1926.

Specifically, inductor system M of the Fig. 6

modification includes the pair of coplanar coaxial inductors l5, l6 positioned in fixed relation and having the same shape, for example a circular shape, but substantially different sizes and adapted to be operated with their axis b-b orientated at a first predetermined angle 51 relative to a line a.a passing through the center of the inductors normal to the plane S of the ground. The axis b-b of the inductors l5, l6 and the line a-a are included in or define a reference plane. As in the previous modification of the invention, the alternating currents flowing through the inductors I5 and I are adjusted in relative magnitudes and phases to produce a magnetic field of which the locus of zero normal component of magnetic intensity is a spherical surface 24 i space. The inductor system M includes the third inductor l8 which is positioned in fixed relation to the inductors i5 and I6 and lies substantially in thesurface 24 with the center of the inductor IS on the line H. The latter inductor is so oriented that the projection of its axis c--c on the aforementioned reference plane forms a predetermined angle or with the line 0-0. The angles 4n and or are so proportioned that the product of the tangents thereof equals 2.

'. tors l5, it and the inductor l8.

For example these angles may each be 54.8 degrees in a preferred embodiment. With this arrangement of inductors, the inductor I8 has substantially zero inductive coupling not only to both of the inductors l5 and it but also to their respective mirror images lEnn, 88m in the ground G. Similarly, both the inductors i5 and it not only have zero inductive coupling to the inductor 58 but also to its mirror image lfim. It may be noted that this nullification of ground coupling between the inductors of the inductor system It' is independent of the height of the inductor system above the ground surface B. As previously explained, when a metallic object A of concentrated dimensions and exhibiting electrical conductivity or magnetic permeability is brought into the vicinity of the inductor system 5', it disturbs the magnetic field existing around the latter and produces coupling between the induc- The particular symmetrical angular relationship of the inductors with respect to the line M, as shown in Fig. 6, has the advantage that it provides by symmetry maximum response to the object A when the latter is directly below the inductor system 86". Additionally, there is the advantage that the orientation of the inductors it, it and it with relation to the ground surface S is not unduly critical because the residual ground coupling is proportional to the square of the angular displacement of the inductors, considered as a unit, from their normal position. In contrast to this, the prior art arrangement of crossed inductors has a ground coupling proportional to the first power of the angular displacement of its inductors relative to the surface of the ground.

From the foregoing description of the Fig. 6 inductor system, it will be apparent that the oblique angle between the axis bb of the inductors l5 and IE on the one hand and the axis c--c of the inductor l8 on the other, this angle being the sum of the angles 1 and 2, has a value proportioned to render the mutual inductance between the inductors 15, I6 and the inductor BB substantially independent of the mass of material G as long as the inductors have a given predetermined orientation relative to the flat surface S of the mass of material.

This angular relationship between the several inductors of the inductor system M may conveniently be considered from another aspect. In the particular arrangement shown, the windings of all of the inductors may be considered to be normal to a common plane which passes through I the center of the inductor system. It willbe apparent that the angle between the projections of the windings of the inductors on this plane has a value 3 equal to the difference between 180 degrees and the sum of 4n and bz. .In accordance with the invention, the angle 3 lies within a range of substantially less than degrees and slightly greater than 70 degrees, a preferred value being approximately 70 degrees.

The inductor system Id of Fig. 6 includes means for carrying the several inductors I5, i6 and id, as a unitary structure, with relation to the mass of material G while maintaining them in a predetermined orientation relative to the surface S thereof. This means comprises a member 21! having one end secured to the several inductors l5, l6 and I8 and having its other end secured to a counterweight 28. By this arrangement a portable unitary structure is provided and the inductors may be carried with relation to a mass of material, such as ground, and maintained in a predetermined orientation with the surface of such mass of material. A carrying handle 29 is secured to the member 21 at an intermediate point. The counterweight 28 serves to maintain the inductor system I4 suitably oriented with relation to the surface S of the mass of material G when the inductor system is carried by the handle 29.

The following table gives several Values of the angles 1, 52 and 3 which are effective to avoid ground coupling between the inductors of the inductor system 3':

Degrees Degrees Degrees 82. 4 82. 6 30 74. 5 75. 5 54. 8 54 8 70. 4 40 72. 8

Inductor 15:

Diameter foot 1 Number of turns Inductor 18:

Diameter foot Number of turns 80 Inductor 16:

Diameter "foot" Number of turns 80 Inductor 15":

Diameter feet 2 Number of turns 1 Inductor 18'':

Diameter foot 1 Number of turns 1 Inductor 16":

Diameter foot V Number of turns 2 While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fail within the true spirit and scope of the invention.

What is claimed is:

1. A unitary balanced-inductor system comprising, a first inductor having a predetermined shape and diameter di and number of turns m, a second inductor having said predetermined shape but having a larger diameter d: and a number of turns us, said second inductor being positioned in fixed coaxial relation to said first inductor and said diameters and said numbers of turns of said first and second inductors being proportioned in accordance with the relation n1/n3= /d3/d1 so that alternating currents of equal magnitudes and opposite phases in said inductors produce a magnetic field which has zero normal component of intensity over a predetermined unbroken three-dimensional surface in space completely enclosing on all sides the smaller of said inductors but completely excluding the larger thereof, and a third inductor having any number of turns and being positioned in fixed relation to said first and second inductors and lying substantially in said surface, whereby said third inductor has substantially zero inductive coupling to said first and second inductors when carrying said alternating currents and in the absence of external concentrated bodies of magnetic or conductive materials but has substantial inductive coupling to said first and second inductors in the presence of such bodies.

2. A unitary balanced-inductor system comprising, a first circular inductor having a predetermined shape and diameter di and number of turns m, a second circular inductor having said predetermined shape but having a larger diameter ds and a number of turns us, said second inductor being positioned in fixed parallel concentric relation to said first inductor and said diameters and. said numbers of turns of said first and second inductors being proportioned in accordance with the relation n1/n2=\/d:/d1 so that alternating currents of equal magnitudes and opposite phases in said inductors produce a mag netic field which has zero normal component of intensity over a spherical surface in space completely enclosing on all sides the smaller of said inductors but completely excluding the larger thereof, and a third inductor having any number of turns and being positioned in fixed relation to said first and second inductors and lying substantially in said surface, whereby said third inductor has substantially zero inductive coupling to said first and second inductors when carrying said alternating currents and in the absence of external concentrated bodies of magnetic or conductive materials but has substantial inductive coupling to said first and second inductors in the presence of such bodies.

3. A unitary balanced-inductor system cornprising, a pair of relatively fixed inductors of the same shape but of substantially different diameters, said inductors being connected in seriesopposing relation and having numbers of turns so proportioned in the inverse ratio of the square roots of their diameters that alternating currents of predetermined relative magnitudes and phases therein produce a magnetic field which has zero normal component of intensity over a predetermined surface in space, and a third inductor having the same shape as said pair of inductors and a diameter equal to the geometric mean diameter thereof, said third inductor being positioned in fixed relation to said pair of inductors and lying substantially in said surface, whereby said third inductor has substantially zero inductive coupling to said pair of inductor when carrying said alternating currents and in the absence of external concentrated bodies of magnetic or conductive materials but has substantial inductive coupling to said pair in the presence of such bodies.

4. A unitary balanced-inductor system comprising, a first circular inductor havin a diameter di and number of turns m, a second circular inductor having a larger diameter d; and a number of turns as, said inductors being positioned in spaced coaxial relation to lie on the surface of a cone and said diameters and numbers of turns thereof being proportioned in accordance with the relation m/na=\/da/d1 so that alternating currents of equal magnitudes and opposite phases in said inductors produce a magnetic field which has zero normal component of intensity over a predetermined unbroken sphericai surface in space having its center at the apex of said cone, and a third inductor positioned in fixed relation to said first and second inductors and lying substantially in said surface, whereby said third inductor has substantially zero inductive coupling to said first and second inductors when carrying said alternating currents and in the absence of external concentrated bodies of magnetic or conductive materials but has substantial inductiv coupling to said first and second inductors in the presence of said bodies.

5. A unitary balanced-inductor system comprising, a first inductor having a predetermined shape and diameter di and number of turns m, a second inductor having said predetermined shape but having a larger diameter d: and a number of turns 11:, saidsecond inductor being so positioned in fixed parallel relation to said first inductor that alternating currents of equal magnitudes and opposite phases in said inductors produce a magnetic field which has zero normal coinponent of intensity over a predetermined surface in space, and a third inductor having a diameter ds and any number oi turns and being positioned in fixed relation to said first and second inductors and lying substantially in said surface, said diameters being proportioned in accordance with the relation z s and said numbers of turns being proportioned in accordance with the relation d d d and second inductors-in the presence of such bodiesi 4 6. A'unitary balanced-inductor system adapted to be carried in a predetermined medium and in proximity to a mass of material, having a substantially flat surface and exhibiting an electrical characteristic different than said predetermined medium, comprising: a pair of relatively fixed coplanar coaxial inductors, included in a common electrical circuit, having substantially the same shape factor but substantially difierent size and having relative positions and numbers of turns effective when said inductors carry alteriii i6 nating currents of predetermined relative magnitude and opposite phase to produce a magnetic field which has zero normal component of intensity over a predetermined unbroken three-dimensional surface in space completely enclosing on all sides the smaller or said inductors but completely excluding the larger thereof; a third inductor fixedly positioned with relation to said pair of inductors and lying substantially in said surface, whereby said third inductor has substantially zero inductive coupling to said pair of inductors carrying said alternating currents; and means secured to said inductors by which said inductors may be carried as a unitary structure with relation to said mass of material and maintained in a predetermined orientation relative to the surface thereof, the angle between the axes of said pair of inductors and the axis of said third inductor having a value proportioned to render the mutual inductance between said pair of inductors and said third inductor substantially independent of said mass of material in the said predetermined orientation of said inductors,

HAROLD A. WHEELER.

REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,577,421 Hazeltine Mar. 16, 1926 1,616,645 Wegner Feb. 8, 1927 1,807,956 Apple June 2, 1931 1,812,392 Zuschlag June 30, 1931 1,865,840 Croft et a1. July 5, 1932 1,905,216 Capps Apr. 25, 1933 1,992,100 v Stein Feb. 19, 1934 2,048,591 Berry July 21, 1936 2,111,210 Ebel Mar. 15, 1938 2,160,356 Fore et a1 May 30, 1939 2,167,490 Ryan July 25, 1939 2,215,605 De Lanty Sept. 24, 1940 2,220,070 Aiken Nov, 5, 1940 2,220,788 Lohman Nov. 5, 1940 2,376,659 Chireiz May 22, 1945 FOREIGN PATENTS Number Country Date 20,656 Australia June 14, 1929 OTHER REFERENCES Geophysical Exploration," Heiland, pp. 819 823, published 1940 by Prentice Hall Inc., N. Y. 

